9.2 Origin of the dinosaurs and birds: Mammals versus Dinosaurs - Gilles Cuny

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From the course by University of Copenhagen

Origins - Formation of the Universe, Solar System, Earth and Life

373 ratings

University of Copenhagen

Origins - Formation of the Universe, Solar System, Earth and Life

373 ratings

The Origins course tracks the origin of all things – from the Big Bang to the origin of the Solar System and the Earth. The course follows the evolution of life on our planet through deep geological time to present life forms.

From the lesson

Colonization of the continents and the Origin of the Dinosaurs and Birds/Mass Extinction Events and Their Causes

In module 9, we will explore how vertebrates colonized dry land. Jesper Milàn will tell you about how this happened and give you examples of some of the first vertebrates that gradually adapted to a life on land. In the following lecture Gilles Cuny will tell you about how the early primitive vertebrates evolved into the highly diverse groups that we see today. He will show you many interesting examples of our distant relatives and discuss many of the processes, which we believe controlled their evolution and diversification. One of the important factors driving evolution is mass extinction events. Gilles will introduce you to the topic and Bent Lindow will give you a detailed look at mass extinction events. What were the causes, what happened and what were the consequences? Many questions remain unanswered but one thing is certain – mass extinctions have had a great impact on the evolution of life on Earth. Without mass extinctions life would have evolved in a completely different way and humans, like most other recent species, would not be here.

9.1 Origin of the dinosaurs and birds: When Life Went Ashore – Jesper Milán13:47

9.2 Origin of the dinosaurs and birds: Mammals versus Dinosaurs - Gilles Cuny16:53

9.3 Origin of the dinosaurs and birds: The end-Triassic Mass Extinction - Gilles Cuny15:10

9.4 Origin of the dinosaurs and birds: Birds are Dinosaurs - Gilles Cuny19:37

9.5 Mass Extinction Events and Their Causes Part 1 - Bent Lindow14:49

9.6 Mass Extinction Events and Their Causes Part 2 - Bent Lindow17:55

9.7 Mass Extinction Events and Their Causes Part 3 - Bent Lindow19:09

9.8 Mass Extinction Events and Their Causes: The Cretaceous-Tertiary Mass Extinction Part 4 - Bent Lindow15:08

Meet the Instructors

Henning Haack
Associate Professor
Natural History Museum
James Connelly
Professor
Natural History Museum of Denmark
Vivi Vajda
Professor
Department of Geology, Lund University
Jon Fjeldså
Professor, Curator
Thomas Gilbert
Professor
Michael Houmark-Nielsen
Associate Professor
Natural History Museum of Denmark
Bent Erik Kramer Lindow
Curator
Natural History Museum of Denmark
Gilles Guy Roger Cuny
Associate Professor
Natural History Museum of Denmark
Ole Seberg
Associate Professor
Natural History Museum of Denmark
Gitte Petersen
Associate Professor
Natural History Museum of Denmark
Tais Wittchen Dahl
Assistant Professor of Geobiology
Arne Thorshøj Nielsen
Associate Professor
Natural History Museum of Copenhagen
Martin Vinther Sørensen
Associate Professor, Curator
Natural History Museum of Denmark
Svend Stouge
Associate Professor
Natural History Museum of Denmark
Danny Eibye Jacobsen
Associate Professor, Curator
Natural History Museum of Denmark
Jan Audun Rasmussen
Associate Professor
Natural History Museum of Denmark
Emily Catherine Pope
Assistant professor
Natural History Museum of Denmark

Hi, my name is Gilles Cuny, and I am curator of vertebrate palaeontology at the Natural

History Museum of Denmark. I would like to tell you about the origins

of modern vertebrate faunas. 250 million years ago, the Permo-Triassic

mass extinction killed up to 50% of the existing families and up to 96% of the species that

lived at that time, inducing therefore a major reorganization of the ecosystems on Earth.

Consequently the beginning of the Triassic was a phase of recovery until the ecosystems

went back to their previous diversity level, but changing the actors.

It took at least 10 million years for the global ecosystem to recover.

In the terrestrial realm, the synapsids, or so-called 'mammal-like reptiles', which

dominated the ecosystems, were replaced by archosaurs, a clade to which belong the dinosaurs

and the crocodiles. The Permian synapsids, here illustrated by

Moschops, had a semi-erect posture, meaning that their limbs are set on the side of their

body, resulting in a rather slow mode of locomotion. During the Triassic, two lineages of archosaurs,

the dinosaurs and the crurotarsans, developed for the first time a more efficient upright

posture, but by different means. In crurotarsans, like for example Poposaurus

from the Late Triassic of the United States it is the girdles that change orientation

so that the femur and humerus are set just below the body.

The limbs being now directly under the body, they are much more efficient at supporting

the weight of the latter. The locomotion is also enhanced as it is no

longer necessary to flex the body sideways when walking, as lizards for example need

to do. Dinosaurs achieved an erect posture in a different

way. The humerus and femur will develop an inturned

head so that the shaft of the bone, and then the rest of the limb, is projected just below

the body. Mammals will subsequently evolve an erect

posture in exactly the same way, although independently.

You can see here the femur of a modern dog. The head of the femur is inturned so that

the shaft of the bone is projected directly under the body.

Erect posture might have been necessary for endothermy to evolve.

Endothermy is the capacity to maintain the body temperature constant independently of

external conditions. Indeed, without an erect posture, the flexing

sideways of the body during walking expands one lung whereas it compresses the other one.

That makes the act of breathing less efficient, and thus reduces the capacity of an animal

to produce enough energy to maintain its body temperature stable.

The main difference between the Permian and the Triassic worlds was therefore the appearance

of faster, more active animals. Mammals and dinosaurs appeared almost at the

same time and the most primitive mammal, Adelobasileus, was found in the Carnian of North America

and is about 230 million years old. It is known from a single incomplete skull,

and it is therefore difficult to know exactly what it looked like, but it was probably quite

similar to a modern shrew, its total length not exceeding 15 cm.

Dinosaurs and mammals have therefore always been contemporary, but dinosaurs quickly reached

supremacy over mammals. Why?

One possible explanation is that the arid climate that predominated at the beginning

of the Norian, 228 million years ago, gave the dinosaurs an advantage over the mammals.

Dinosaurs, like crocodiles and birds, had a metabolism that did not require as much

water as that of mammals. Their skin, contrary to that of mammals, was

impermeable and they did not sweat. Also, mammals produce liquid urine, causing

another source of water loss, whereas dinosaurs produced semi-liquid urine, forming a �paste�

like in modern birds. In a Norian world where desert conditions

were more common than today, dinosaurs would therefore have a serious evolutionary advantage

over mammals. Once the domination of the dinosaurs was established,

mammals were not able to rise to a dominant position until another mass extinction gave

them such an opportunity. The first stem-mammals were small, mouse-sized

animals so their fossil skeletons are quite rare.

However, their small teeth, much more prone to fossilization because more mineralized

than bones, are often recorded in many areas of the world during the Mesozoic, so that

the history of these little furry animals is quite well known, although it is sometimes

difficult to know what they looked like. The first representatives of the two major

modern groups of mammals, the pouch-bearing marsupials, or Metatheria and the placentals

without pouch, or Eutheria first appeared between the Middle Jurassic and the Early

Cretaceous, some 65 million years after the appearance of Adelobasileus, the first stem-mammal.

Both the first marsupial and the first placental have been found in China.

As I said, because of their small size, fossil mammals are more often known from isolated

teeth than from complete skeletons, but fortunately exceptions exist that allow us to understand

the main steps of their evolution. Here is a reconstruction of the oldest placental

known to date. Its name is Juramaia, and the anterior part

of its skeleton has been found in the Middle Jurassic of China.

It was a small animal, not exceeding 20 cm in total length, but it demonstrates that

modern types of mammals appeared some 165 million years ago.

However, it would be wrong to think that more primitive mammals, without living relatives,

did not evolve during the age of dinosaurs. Again, some more or less complete fossils

reveal the existence of surprisingly specialized stem-mammals.

Fruitafossor for example is a fossorial stem-mammal, the lineage of which appeared well before

the separation between the placental and marsupial lineages.

A relatively complete skeleton of this little animal, the weight of which did not exceed

6 grams, was found in the Late Jurassic of North America.

Its peg-like teeth devoid of enamel are very similar to those of a modern armadillo, indicating

a specialized diet made of colonial insects. It possessed also large front limbs, well

adapted for digging, so that it was able to break into termite mounds.

Although primitive, Fruitafossor shows therefore a high degree of specialization, both for

digging and in terms of diet, challenging the idea that early mammals were restricted

to unspecialized niches. Another good example of a specialized stem-mammal

is the gliding Volaticotherium from the Middle Jurassic of Asia.

Its gliding membrane, or patagium, was supported by the limbs and the tail.

It is the first known gliding mammal, although it is only distantly related to modern flying

squirrel. The shape of its teeth indicates that it was

a specialized insect eater. Castorocauda is a semi-aquatic stem-mammal

with a broad tail similar to that of a modern beaver.

It was first found in the Middle Jurassic of China and is therefore some 165 million

years old. Fossilized impressions of some webbing are

present between the toes, reinforcing the idea it was well-adapted to an aquatic mode

of life. Its sharp teeth also suggest it was piscivorous,

eating mainly fish. It is the very first mammal adapted to a semi-aquatic

mode of life in the fossil record, and such a specialization will not reappear in mammals

before the Eocene, 56 million years ago. The forelimbs of Castorocauda were robust

and adapted for digging as well. They are quite similar to those of the modern

Platypus, which both digs and swims. However, this is again a convergence as Castorocauda

is a stem-mammal only distantly related to Platypus.

Another interesting feature about this critter is its size.

It could grow longer than 40 cm and it is actually the largest Jurassic mammal known

to date. Repenomamus from the Early Cretaceous of China

is about 125 million years old. It is thus younger than Juramaia, Fruitafossor,

Volaticotherium or Castorocauda. It could exceed 1 m in length and its overall

appearance is quite reminiscent to that of a modern Tasmanian devil.

One fossil specimen displayed some remains of a juvenile of the small ornithischian dinosaur

Psittacosaurus at the level of its stomach. Repenomamus was therefore an active predator,

which may also have scavenged dinosaur carcasses. It is the largest known Mesozoic mammal, and

it was indeed larger than some of the predatory theropod dinosaurs that could be encountered

at the same time in China. So, the very first mammals evolved in the

Late Triassic, at the same time as the dinosaurs, from one of the synapsid lineages that survived

the end-Permian mass extinction. However, the rather arid climate at the time

was more beneficial to dinosaurs than to mammals and the former quickly outcompeted mammals

and reached dominating position in the Late Triassic ecosystems.

We shall see, however, in the next video that it was not so easy for the dinosaurs.

That does not mean, however, that mammals did not evolve during the age of dinosaurs.

It is often said that Mesozoic mammals were all small nocturnal mouse-like animals.

This is clearly an oversimplification. Even if mammals did not reach dominant positions

in Mesozoic ecosystems before the demise of dinosaurs at the end of the Cretaceous, they

nevertheless diversified during the Mesozoic, becoming gliding, digging or aquatic.

Most of the actors of this first diversification event are not directly related to modern mammals

though. The first representatives of the two main

modern lineages, the marsupials and the placentals, appeared anyway as early as the Middle Jurassic,

165 million years ago. The Late Triassic was also the time when many

other lineages that characterize our modern vertebrate faunas appeared.

Among them, we found the lizards. The first representative of the lizard and

snake lineages, the Lepidosauromorpha appeared in the Carnian, 225 million years ago, and

belongs to the Sphenodontia, a lineage represented nowadays by a single survivor, the Tuatara,

or Sphenodon, from New Zealand that you can see on this picture.

The first sphenodontians were small animals, between 15 and 35 cm long.

Their teeth suggest that some were insectivore, others herbivores.

It took, however, nearly 100 million years before the most derived members of the Lepidosauromorpha,

the snakes, appeared in the Early Cretaceous. Another modern clade to appear in the Late

Triassic is the turtles, although as we shall see their origin is still quite mysterious.

Turtles have the most unusual body plans of all amniotes, meaning the tetrapods that lay

eggs on the ground. As we can see on this fossil from our collection,

the body of a turtle is protected by a ventral plastron and a dorsal shell which is fused

to the ribs. That means that the scapula, the bone making

your girdle that you can feel here, which normally is outside the rib cage, is in the

turtle inside the rib cage and this is really a major modification of the general organization

of a vertebrate. These highly unusual tetrapods appeared for

the first time in the Norian (215 million years ago).

One of the oldest turtles, the emblematic Proganochelys, is known in Europe, North America

and Thailand, indicating that these animals were very quickly successful in many parts

of the world. Proganochelys was very similar to modern turtles,

except that it could not retract its head under the carapace, and instead the neck was

protected by pointed plates. It also possessed teeth, as well as dermal

plates covering the extremity of its tail, making it a defensive weapon.

It was a rather large animal, measuring about one metre in length.

Later turtles will develop the capacity to retract their heads under the carapace when

they are threatened by danger. The way they do it defines two groups today.

The pleurodires do it by making a sideways bend in the neck, whereas the cryptodires

make a vertical bend. It is always problematic to try to decipher

the phylogenetic affinities of a group as specialized as the turtles.

The characteristic that actually makes a turtle, that means the carapace, is unlikely to be

present in the putative ancestor. The problem is then to understand how the

carapace appears to identify the potential sister group of the turtles, and then to understand

how they originate. Recent discoveries have revived the debate.

A fossil found in the Norian of New Mexico, Chinlechelys shows an animal with a very thin

dorsal shell not yet fused to the ribs. This animal could therefore be potentially

very important to help us solve the relationships of the turtles with the other amniotes.

It would nevertheless suggest that the dorsal part of the carapace was the first to appear

and resulted from the fusion of small dermal bones covering the body of the putative ancestor

as we can see on this picture. The covering of the neck and tail will subsequently

disappear. Another fossil found in the Carnian of Guizhou

Province (South China) is Odontochelys, and it offers a rather different story.

The dorsal carapace is hardly developed while the ventral plastron is fully developed, which

is exactly the contrary of the condition observed in Chinlechelys.

Moreover Odontochelys is aquatic whereas Chinlechelys is fully terrestrial.

So was the earliest turtle aquatic or terrestrial? Odontochelys is now widely accepted as the

sister-group of turtles, and an aquatic phase in the early evolution of turtles appears

therefore more likely. However, there are still quite a lot of discussions

in the scientific community as where to nest the turtles: at the base of the diapsids lineage,

a clade grouping the lizards, crocodiles and birds, or among the anapsids, close to the

more primitive pareiasaurs like this Bradysaurus from the Permian? In the current state of

our knowledge, it is still difficult to answer this question with certainty.

Many questions remained to be solved before we can fully understand the evolution of life

on our planet. We have looked in this first video at how

some of the major groups of modern tetrapods: mammals, lizards and turtles appeared in the

Late Triassic. Next time we'll have a look at the origin

of the crocodiles as well as modern bony fishes and modern sharks.

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